Why did Bud's mom run away from Mr. Calloway?

The exact reason that Bud's mother ran away is not given. From the text, we can infer that Herman had high expectations.... and that he pushed them on his daughter.... not allowing her to make decisions for herself.

From the text:

He used to crow about how his mother and father had been born slaves and how now it was only two generations later and the Calloways had come so far and worked so hard that one of them was actually going to be a teacher. "It was his dream, not hers--not yet, anyway--and he never gave her time to pick it for herself. The more he pushed her, the more she fought him. Finally it got to be too much and she left. We think she ran off with one of Herman's drummers.